mtd: nand: omap: Update DT binding documentation

Add compatible id and interrupts. The NAND interrupts are
provided by the GPMC controller node.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Tony Lindgren <tony@atomide.com>